1. Field of the Invention
This invention pertains to the general field of protective substances used to coat the cutting surfaces of metal-working tools in order to improve their abrasive performance and longevity. In particular, it provides a new way of improving the performance of grinding tools by applying a thin layer of titanium nitride, titanium carbide, or other equivalent coating over the abrasive material used to coat the steel substrate of grinding tools.
2. Description of the Prior Art
Abrasive substances are used for cutting, smoothing and polishing the surface of other hard materials. Naturally, in each instance the abrasive used must be harder than the surface it is working on in order to ensure proper performance and durability. Therefore, materials known for their high degree of hardness, such as diamond, aluminum oxide and silicon carbide, have long been used as the cutting edge of tools to improve the quality of grinding, sawing, lapping, machining and drilling. These materials reduce production time and costs, and facilitate the maintenance of consistently high tolerances with little or no effect on the metallurgical integrity of the final product.
Typically, the working surface of a tool made of steel or other metallic substrate is coated with a layer of harder and more abrasion-resistant material by chemical bonding, electrometallurgical deposition, or other methods of application. Thus, in addition to the properties of the abrasive material itself, the performance of the tool is affected by the quality and durability of the bonding between the abrasive particles and the tool substrate. U.S. Pat. Nos. 3,464,804; 3,645,706; 3,650,714; 3,859,057; 3,879,901 and 3,929,432 describe several processes and products directed at improving both of these critical characteristics in grinding tools.
Because of its hardness, for a long time diamond was the preferred abrasive used for grinding very hard superalloys and composite materials, but it could not be used effectively on steels and other ferrous surfaces because of its tendency to react and be absorbed into such surfaces at the temperature and pressure conditions normally occurring in the grinding and machining process (a characteristic known as carbon solubility potential). A great improvement was achieved by the development of the synthetic superabrasive substance cubic boron nitride (CBN), marketed by the General Electric Company under the trademark "Borazon," which exhibits abrasive properties comparable to diamond and can be used effectively for grinding hardened steels. In fact, the performance of CBN is far superior to that of aluminum oxide, the conventional abrasive used on hardened steel tools.
The manufacture of most CBN grinding wheels currently found in the industry is based on four major types of bonding techniques to incorporate the CBN cutting surface into the wheel's metal core, which generally consists of steel. These techniques include resin bonding, metal bonding, vitreous bonding and electroplating. The resulting CBN-coated grinding wheels are now routinely used (generally referred to as Borozon wheels) in the machining of hard ferrous metals, cast irons, and nickel-base and cobalt-base superalloys. They can grind more material, to a higher degree of accuracy and at a lower cost, than any other abrasive. In fact, the limiting factor in the life of such tools is typically determined not by wear on the cutting surface but by its break-down and separation from the metal core resulting from failure of the bonding layer.
Therefore, any process, material, or technique that might result in the strengthening of the bond of the CBN or any other abrasive material with the supporting substrate would be of great usefulness and commercial value to the industry. The present invention deals with a new approach to strengthening the bond between the metal core (substrate) of the grinding wheel and the CBN layer on its cutting surface. The approach is based on the physical vapor deposition of a protective layer, such as titanium nitride, on the CBN-bonded surface. In addition, this invention also shows that the same process can be used to coat standard diamond tools with titanium nitride or equivalent protective layers to greatly reduce their carbon solubility.
Nitrides, carbides and other equivalent substances have been used for some time in thin coatings to provide wear protection to moving parts of equipment and machinery. This protection has become particularly critical in recent years in automated production processes, where machine break-downs caused by wear of individual parts are a significant component of overall costs. Therefore, thin layers of these materials (especially titanium nitride) have proven very successful in reducing operating costs. They are extremely hard, abrasion-resistant, and adhere well to the supporting surface even in very thin layers, which is very important for the dimensional tolerances allowed for coated parts. Such protective materials have not heretofore been used, though, to coat abrasive particles (such as CBN and diamond) that themselves are bonded to a metallic supporting substrate.
In general, the deposition of these thin layers of coating materials has been achieved at relatively high temperatures by chemical vapor deposition (CVD) and by physical vapor deposition (PVD) of a layer of the material (for example, titanium nitride) over the substrate tool. Typically, titanium nitride is deposited by chemical vapor deposition at temperatures in the order of 1,200.degree. to 1,700.degree. F. and by physical vapor deposition at 600.degree. to 1,200.degree. F. When applied to nickel-plated Borozon or diamond wheels, these temperatures tend to affect the integrity of the substrate and the stability of the abrasive-steel bond. This invention is based on the discovery that any of the several procedures known in the art as the physical vapor deposition (PVD) process can be used successfully at relatively low temperatures to produce Borozon tools coated with titanium-nitride or equivalent protective coatings for improved durability and efficiency.
This invention is further based on the recognition that titanium nitride and equivalent coatings may be used effectively also for protecting steel tools and wheels coated with diamond particles bonded to the steel substrate by means of electroplated nickel. The titanium nitride layer has shown to have the same bond-strengthening characteristics previously seen in Borozon-coated wheels. In addition, because of the presence of the buffer layer provided by the deposition of titanium nitride, the carbon solubility potential of the diamond is reduced, making it economical to use diamond tools for grinding steels and other ferrous materials.
Although the invention refers more particularly to titanium nitride, other equivalent coatings that utilize metals such as titanium, zirconium, boron and aluminum or alloyed metals like titanium-aluminum, titanium-zirconium, and chromium-aluminum in conjunction with gaseous carbon, oxygen, nitrogen, or mixtures thereof, may be equivalently utilized by those skilled in the art to produce a protective layer deposited over the abrasive medium by physical vapor deposition methods.